1. Field of the Invention
This invention relates to fabrication methods for field effect transistor (FET) gates, and particularly to FETs with sub-micron gate lengths that are subject to a degradation in their yield when the unused gate metallization is lifted-off during fabrication.
2. Description of the Related Art
A functional diagram of a typical FET is given in FIG. 1. The device consists essentially of a source 2, a drain 4 and an intervening gate 6. With a voltage differential between the source and drain, the signal applied to the gate controls the flow of current 8. Below a particular negative gate voltage level, referred to as the pinch-off voltage, the flow of current is terminated.
FIG. 2 is a sectional view of a high electron mobility transistor (HEMT), which is a form of FET to which the present invention is particularly applicable. HEMTs are characterized by a very high transconductance, very high frequency operation and very low noise. They are fabricated from material systems such as AlGaAs/GaAs or AlInAs/GaInAs/InP.
The HEMT 10 is shown as including a source 12, drain 14 and an intervening gate electrode 16. A highly doped, wide bandgap semiconductor material 18 is inserted between an undoped semiconductor layer 20 which forms the conducting channel, and the gate electrode. When properly designed, a discontinuity in energy bandgaps between the two semiconductor materials results in a free charge transfer from the highly doped, wide-bandgap semiconductor 18 into the undoped, lower bandgap channel semiconductor 20. Charge accumulates at the interface of the two semiconductors and creates a two-dimensional electron gas (2 DEG), allowing current to flow between the source and drain electrodes. The amount of charge in the 2 DEG is controlled by the potential applied to the gate electrode. Since the charge flows at the interface between the two semiconductor materials, but is confined in the updoped channel semiconductor 20, there is very little impurity scattering and an extremely high charge carrier mobility results.
Sub-micron gate length HEMTs and other FETs are presently highly promising technologies for microwave and milimeter-wave applications. (The gate length is defined as the gate dimension in contact with the semiconductor substrate in the current flow direction between the source and drain.) However, as gate lengths have been reduced, there has been a corresponding increase in the proportion of devices that do not pinch-off. Yields have accordingly fallen, lowering production rates and increasing costs.